The technology relates to cellular radio communications, and in particular, to mitigating interference between base stations and their served devices.
The constantly increasing demand for higher data rates in cellular networks raises the challenging question for operators of how to evolve their existing cellular networks so as to meet this requirement for higher data rates. In this regard, a number of approaches are possible: 1) increase the density of the typical existing macro (larger, higher power) base stations, 2) increase the cooperation between macro base stations, or 3) deploy smaller lower power base stations as an overlay to the macro cellular network in areas where high data rates are needed within the macro network topology.
In 3GPP, the last option (3) is commonly referred to as a “Heterogeneous Network” or “Heterogeneous Deployment.” The lower power base station nodes are referred to by a variety of names including “micro”, “pico”, “femto” or “home” base stations (HBS) depending on the transmit power and targeted coverage of the low power nodes. Specifically, the deployment of femto base stations, e.g. Home NB (HNB) in 3GPP UTRA or Home eNBs (HeNB) in LTE, can lead to much greater local coverage and throughput, e.g., for in-building users.
Often, HeNB's are deployed using a Closed Subscriber Group (CSG) in order to restrict authorized users of the HeNB to an identified and approved set of users or UEs (user terminals). When CSG HeNB's are deployed as an overlay to an existing macro cellular network deployment, macro network UEs (MUEs) that are not part of the CSG can adversely affect the CSG HeNB. FIG. 1 illustrates two possible interfering examples. In a first example, interference can occur if MUE (non-CSG) 104 is near the edge of the macro cell coverage 111 of serving cell base station macro eNB 101 and receiving a relatively weak downlink (DL) signal, while simultaneously being close to a HeNB (serving cell 103) and receiving a relatively strong interfering DL signal from the CSG HeNB 102. This will lead to reduced DL throughput for the MUEs and possible loss of coverage depending on the strength of the CSG HeNB interferer. Though a similar situation can occur between HeNB's in the same macro overlay network, the near-far problem between a HUE 107 served by one CSG HeNB 102 versus a HUE served by a second CSG HeNB is likely to be less severe. In a second example, on the UL, a similar situation can occur in which a non-CSG MUE 104 UL signal and/or non-CSG HUE 105 UL signal can cause interference to CSG HeNB 102 UL if they are in relative close proximity to CSG HeNB (serving cell 102).
In a CSG HeNB deployment, one solution to this interference problem for the non-CSG MUE or HUE is to handover the MUE or other HeNB HUE to the CSG HeNB and thus let the non CSG MUE or the other HeNB HUE be served by the CSG HeNB. However, in a CSG HeNB deployment, such a handover for non-CSG MUEs is in general not possible because the CSG HeNB cannot directly communicate with the macro eNB since, in general, HeNBs are not linked to the macro radio access network via S1 or X2 interfaces.
Another potential issue is that another HeNB in the vicinity of the CSG HeNB may have the same physical cell identifier (PCI) as the CSG HeNB which may cause not only data channel interference to the HUEs of the CSG HeNB, but incorrect control channel signals that may result in a complete outage of the HUE due to incorrect access grant, resource block assignments, incorrect power control, incorrect ACK/NAK signaling, or a variety of other control channel errors.
A number of potential approaches may be used to mitigate the interference to non-CSG MUEs within the coverage area of CSG HeNB's. One possible approach employs some form of autonomous power control of the CSG HeNB transmit power based on the received signal power from the MUE or on the strongest co-channel interferer as seen by the HeNB. These approaches can reduce the interference and outage as seen by the MUE. However, the HeNB has no direct knowledge of the radio bearers (RBs) that the MUE is scheduled on or the actual level of interference that the MUE is receiving since the received signal strength received from the MUE in the UL at the HeNB is different from the strength of a DL signal received from the HeNB at the MUE, e.g., because of different transmit power levels of base stations and UEs which may also be dynamically controlled. As such, it is difficult for the HeNB to accurately adjust its power to sufficiently reduce the interference to the non-CSG MUE without overly reducing the received signal power as seen by its own HeNB UEs, and thus, overly penalizing the throughput of the CSG HeNB UEs (HUEs).
A second possible approach is to have the HeNB avoid scheduling its UEs on the same RBs as the MUE that is in close proximity to the HeNB. This approach can effectively mitigate the interference but reduces the capacity of the HeNB. Moreover, the HeNB generally does not have a priori knowledge of the RBs that the MUE will be scheduled on.
Another problem in the CSG environment or in an environment with a low power radio node is that strict physical cell identifier (PCI) planning is difficult because a CSG HBS can be moved from one location to another. Moreover, there can be a very large number of CSG HBSs in a small coverage area, e.g., in a large apartment complex. Given that usually there are only a limited number of PCIs (e.g. 504 in LTE), the reuse of PCIs in closely-placed CSG HBSs may lead to severe interference in the network deteriorating the network performance.